This course is an introduction to ecology and ecosystem dynamics using a systems thinking lens. Through a case study on Mozambique's Gorongosa National Park, learners will explore how scientists study ecosystems, and investigate the complex array of factors that inform management efforts. At the end of the course, learners will be able to grapple with real-world conservation questions, such as whether an ecosystem can recover from anthropogenic disruption and what role humans can, and should, play in that recovery.

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LB

A great summary of the challenges being faced in the conservation science community at the moment. From an Environmental Science background it was great to learn about new case studies.

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Apr 29, 2020

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Wow. This was an amazingly informative course. The content I timely and relevant to all humans not just those of us involved in ecology and conservation. I am just sad that it is over.

From the lesson

Can an Ecosystem Recover?

We begin in Gorongosa National Park, Mozambique, and pose the question: Can this ecosystem recover after a 15-year civil war? To answer this question, learners must first consider what they need to know—what are the parts that make up this ecosystem, and how do they interact and work together? How do ecosystems react to disruption? How do we know? We will begin to explore the ecosystem as a dynamic whole rather than as a collection of parts, considering how changes might affect the system in a variety of ways. This application of a systems thinking lens to understanding ecosystems will be a common theme throughout the course.

Taught By

Ana Luz Porzecanski

Transcript

Welcome to HHMI'S 2015 Holiday Lectures on Science. This year's lectures, patterns and processes in ecology, will be given by two leaders in ecological research, Dr. Robert Pringle and Dr. Corina Tarnita of Princeton University. Their work combines field ecology, experimental biology, and mathematical modeling to understand how plant and animal communities are organized, how natural patterns arise, and the effects of human impacts, including species extinctions and climate change. The first lecture is titled Africa's Savanna Ecosystems. And now, Dr. Robert Pringle. Hi everybody. Thanks for coming out. I was so excited to be here with you and talk a little bit about ecology, which is the field that we are in and our main life passion. So, looking forward to meeting as many of you as possible over the course of the day, getting a chance to chat. Now, I want to start off by showing you this photo, and ask yourself what you see in this photo. And obviously, there are some very conspicuous elements here, right? There's a lion in the foreground, and the waterbuck in the background. But this image is actually a very nice representation of an African savanna. And there are several less conspicuous features that I want to draw your attention to. So, one is it, obviously we've got the grassland in the foreground here, but in the background you'll see a range of trees. And in the foreground you got green forb or weed in the middle of the grasses, right? And there, off to the side, you've got a white cattle egret hanging out amongst the waterbuck. So, you might ask yourself, if you're an ecologist you're looking at this photo, you're asking yourself, what are the processes that produce these patterns? Okay. So, those are words that we're going to be using a lot today, process and pattern. And by process we mean, sort of, the mechanisms that generate observable, repeatable phenomena, right? And the patterns are those observable, repeatable phenomena. Now here's an example of a literal pattern. These are clumps of trees in a grassland matrix. Also in Mozambique, very close to where that lion was sitting. And from the air you look at this and you see the pattern immediately jumps out to the eye, right? It is a very regular even configuration of tree clumps. And so again, as an ecologist you might ask yourself, what is it that causes this pattern, how does it arise, how is it maintained? We tend to think as humans, we tend to think of nature as being fairly disorganized, chaotic, random, but in fact it can be very, very highly structured and well organized. The last set of terms that I want to introduce you guys to relate to scale. And that's another thing that ecologists think a lot about spatial and temporal scales. So, in terms of spatial scale, the lion and the waterbuck that you just saw, the background of that photo is effectively the habitat for those species, right? Habitats are nested within larger ecosystems or landscapes. So, in this case it's the greater Gorongosa ecosystem, that is home to this lion, waterbuck, and cattle egret. And, of course, larger ecosystems and landscapes are embedded within biomes. And in this case, specifically, the African savanna biome. Now, you might have heard this term, biome. This is something that gets a lot of attention in biology textbooks, and most scientists view biomes as being largely determined by climatic factors. So, here you see a range of biome types that may be familiar, there is the Arctic tundra way up north in blue, and there is the Amazon rainforest and the Congo Basin rainforest in green, also tropical forest over here in Southeast Asia, big deserts in Australia and the Sahara in North Africa, and the Southwestern United States, okay? And in each of those cases, I think, fairly think about the biomes as being driven by climate, right? Deserts occur where it's dry. Rainforests occur where it's really wet. Tundra occurs where it's cold. But I want, today, to talk to you guys about one particular biome and that's savanna, okay? And savanna is globally important in terms of its extent, it covers about 20 percent of the global land surface, and it covers more than half of Africa, which is a continent that I'm particularly in love with. It's got almost 40 percent of the land area of South America. And there are some things about savanna that make it particularly interesting, and worth spending a whole morning on with you guys today. One of them is that savanna is that easy to define, but I would say hard to explain, and I'll explain what I mean by that in a minute. In easy definition, for savanna is an area of intermediate tree cover with grasses. So, this map shows the distribution of areas in Africa with tree cover ranging between 11 and 80 percent, right? So it's not so few trees that it becomes grassland or desert, but not so many that it becomes forest. And additionally being globally widespread, and important in terms of just geographic distribution, savannas are important for several other reasons. One is that they're the last remaining home of diverse large mammal wildlife, like these zebras here. And you guys may know that even here in North America at one time there were incredible spectacular megafauna, a woolly mammoths mastodons, giant ground sloths, these kinds of things, right? But those are all extinct now. And Africa is really the last place on earth where you have intact assemblages. And by intact I mean no species have gone extinct. They're are also important because they're the source of livelihood for tens of millions of people, right? Everything from these pastoralists here with their cattle, Maasai pastoralists, to the people driving the cars and arranging the tours for these international tourists, who have come to see lions down here in the bottom left underneath the tire of the Land Rover, and then go out and go [inaudible] , okay? So, economically they're important, globally they're widespread. And the rest of today really is going to be about why ecologically they're so interesting and important. Now, I gave you a sort of a brief attempt definition for savanna in a previous slide. A more concise version would be that savannas are areas where trees and grasses coexist. All right. And you'll see this is, I'm not taking you into the middle of a savanna. This is again Gorongosa National Park in Mozambique, and you can see in every photo here, that you've got a grass understory with a tree canopy, right? And it's that coexistence of trees and grasses that make savanna. And you can think about, right? When you go into a deep forest with close canopy, you don't find grasses. When you go into grassland, you don't find trees. So savanna is in between them. Now, that actually poses, that sounds simple, right? It sounds straightforward that tree-grass coexistence. But ecologically, it's not necessarily a guaranteed outcome. One reason is that these two types of plants are going to be competing with each other. They are able to sort of deal with that to some extent by having different rooting zones. The trees can access water that's deeper in the soil profile than the grasses can, as you can see in the photo. But nonetheless, trees compete with grasses for light by shading them out, and grasses can compete with trees, especially with young trees, right? And I want to draw your attention to that little young tree. There's the little seed, the little seedling interspersed in the grasses there. So, that's the tree that is particularly vulnerable to competition with grasses, okay? So, that tree is competing with grass for light, it could get smothered for water, it could dehydrate. And it's competing in another way, which is that periodically in the savanna systems fires come through. And grasses particularly they're flammable, especially when it gets dry. And the annual fires that go through many savanna ecosystems can kill many tree seedlings. So, these are ways that trees and grasses compete. And the question is now, how do they coexist given that they're competing? So, any time you want to think about the distribution of different kinds of plants, you want to start thinking about the rainfall regime. Now, this graph from a 2005 paper by Mahesh Sancaran, an ecologist, and it shows the amount of tree cover on the y-axis as a function of rainfall on the x-axis. And rainfall goes from 0 to 1200 millimeters per year, and each of these points represents a geographic spot in Africa, right? And so, you can see that when there's no rainfall, there's no tree cover, and that kind of stands to reason, right? I think you guys agree with me on that. But what I want to sort of focus more on is this area underneath this curve. So, this curve basically that the line here is defining the maximum amount of tree cover. And you can see that the maximum amount of tree cover increases until you get to about 700 millimeters of rainfall. And then there's a threshold, which is really cool. Ecologist always get really excited when they find some kind of threshold. And at the threshold, the graph flattens out at around 75, 80 percent tree cover. So, the message here is that at relatively high rainfall, intermediate rainfall from 700 millimeters per year on up, you can get a forest. And by forest we're saying things with 80 percent tree cover or greater, you can get it, but for the most part you don't. Almost all of those points on the graph at high levels of rainfall, have less tree cover than is theoretically expected based on the amount of rainfall alone, right? So that means that at really high levels of rainfall, what you can get is this close canopy woodland. At a very low rainfall, you get grassland and desert. But a lot of sites, most sites in Africa fall somewhere in between, and beyond that, even sites where there's high rainfall often do not reach their maximum capacity in terms of tree cover. And I alluded briefly to the role of fire in a previous slide. Fire is thought to be one of the major, if not the major, factors explaining the points on the right-hand part of that graph. Why do trees growing in areas that otherwise have sufficient rainfall to get close canopy tree cover, why don't they? This is just a frequency histogram showing the number of sites. This is basically on the y-axis, just the count of different areas. And on x-axis is the percent tree cover of those areas. And you see that when fire is present, there is this nice kind of bell-shaped curve with a median mode around 40 percent tree cover, okay? So in fire-prone ecosystems, you don't get super high tree cover. You tend to get intermediate cover. And when fire is absent, you tend to get really high tree cover, okay? And these are areas I should say with at least a thousand millimeters a year rainfall. So, take-home message of this graph is that for those relatively high rainfall areas, fire is the main factor that is keeping tree cover down, okay? So, now, I've discussed a couple what we call abiotic factors in the sense that they're nonliving that constrain tree cover in savannas. And the first is rainfall, and the second is fire. There's another one that I don't have time to talk about, which is soil nutrients, fertility of the soil. But I want to make the argument to you guys that biotic factors, biological interactions are equally crucial, if not more so, in defining the savanna biome and defining savanna ecosystems as our abiotic factors. So, the biotic factors would include things like competition and facilitation between plants. You're going to hear a lot about that in lecture two. They would include herbivory, which you're going to hear a lot about in lecture three. They would include ecosystem engineering, which is something you also hear about in lecture two. And, of course, predation, apex predators increasingly thought to be crucial to how ecosystems work. So, I want to try to get across that these biotic factors, they sound like anybody who studied a little bit of ecology, all those biotic factors sound really important. Yeah, we study competition, predation, all these things because they really matter. But in savanna ecology, really, they've got and I would say they've gotten short shrift. In this graph is just from a 2008 paper, where they looked at the literature and tried to quantify the relative importance of different variables then explaining the properties of savannas. And you see these abiotic factors from rainfall being most important to fire, soil nutrients, phosphorous and nitrogen, and clay content get assigned the most importance as factors explaining savannas. And three biotic interactions, specifically three types of large mammalian herbivore, elephants; browsers, which eat trees and forbs and shrubs; and grazers, which eat grasses, are consigned to relatively lower importance. All right. And the argument I'm going to try to make, and Karena I think will try to make over the course of today is that this may not be wrong, but we feel it's incomplete, and that the biotic interactions are really crucial to understanding these systems. Now, you guys might be wondering why I've got such a BMI bonnet about this. Why am I so convinced that these biotic interactions are so important? And to kind of illustrate to you why that is, I want to just share a brief story of research that's come out of my lab specifically research done by my graduate student Josh Gaskin. And what Josh was interested in was Gorongosa National Park that I showed you the movie clip of has a tragic history. And you guys are going to hear about this and understand exactly what happened there. But during the 1970s, '80s and especially '90s, almost all the large wildlife was wiped out. And Josh wanted to know how did that impact the tree cover in the National Park. And so he did a clever thing. He found some declassified CIA spy imagery that had been taken of Mozambique, and he was able to get his hands on that and compare it to modern imagery from 2012. And the results are pretty striking. So this is a proportion of tree covers a proportional cover of woody plants in 1977 and 2012, and you see that it's gone up quite a bit. In fact, across the whole of Gorongosa National Park, it's increased by more than 30 percent. But that sounds less impressive than it is because in some areas, the increase has been more than double, okay? And if you want to look on the map, you can see each of the every point on this map that is green is an area where grasses transitioned into trees between 1977 and 2012 over that 35 year interval, right? And this we attribute to the loss of large herbivores like elephants and others from the ecosystem. So, it is my argument to you guys that if we want to really understand these systems, we're going to have to look at them through the lens of biotic interactions, okay? And we can do that both figuratively and literally. This is a footage from a critter cam, which is basically a GPS collar with a video camera attached. And it's giving us a very up-close and personal sense of what it's like to be a water bug in Gorongosa National Park. And we can do this now. This is a fun technology. It's actually like a beta testing version from National Geographic, and they let us test it out on some of our critters there. And so we can, in addition to being super fun, to just see now be able to kind of put yourself into the body of a water bug or in the eyes anyway and see kind of what's going on there. It actually can yield usable scientific data, really important kinds of scientific data. They don't get any other way, okay? So, the major players we're going to be talking about today in terms of biotic interactions or plants obviously, they are surprisingly perhaps termites, which are absolutely fundamental to African savannas, like if I were to pick the single most important type of organisms in savannas, it probably be termites. Large herbivores, you're going to hear a lot about them. Carnivores, you actually going to hear surprisingly little about them today just because we didn't have enough time. If we had had one more lecture, we would have had a whole lecture about predation, but we do have some in there. And, of course, people, right? And people, you can't understate the influence that people have on ecosystems globally but on savannas in particular because humans evolved in the savannas, and humans have been co-evolving with savannas ever since. So, that influence goes way back in history. If we want to organize those key players into a trophic pyramid that indicates where they are in the food chain, we do it like this. We put people at the top, and then the apex predators like lions, leopards, cheetahs, wild dogs, hyenas, bat-eared foxes, and so forth at the predator trophic level. Then we'd have herbivores there at the yellow bar, and plants at the bottom. And here we have in the soil, we have various nutrients and what we call resources, water, and nutrients. And then I brought termites in on the side because they're down there in the soil having a crucial effect. So, again, what we're going to be focusing on today is explaining how biotic interactions are involved in processes that generate the salient patterns that I think we all recognize and appreciate about savannas, okay? So, I'm not going to get into detail about what each of these things are because they're actually each from a future lecturer, and we're going to get to them through the course of the morning. But each of these major groups of organisms is going to make an appearance. So, what I want you to take away from this, savannas are globally important ecosystems, ecologically, and economically. And the savanna biome is kind of unusual and not being determined by climate alone. And it is an area where scientists are actively debating, actively debating the causes of what causes the savanna, what determines where savanna is, and what determines its properties, like it's distribution and abundance of species. And, lastly, whereas the importance of the abiotic factors is well established, we really need more work on understanding the role of biotic interactions, so species interactions, and other interactions between species. So, that I will take a few questions. Yeah. Is there any one specific factor that causes the hard threshold that 80 percent tree cover, or is it a confluence of multiple abiotic and biotic factors? It's a great question. I think it's probably a pretty hard threshold that's based on just water availability. Trees require a lot of water to grow. And to get an ecosystem with really close canopy tree cover, there's just not enough water in semi-arid ecosystems with less than 700 millimeters per year. So, it's really that once you exceed that 700-millimeter threshold, then there's the potential, right? That's the first time that you've sort of you crossed that potential in just in terms of sheer water availability. And then whether you actually achieve that potential or not is a function of the confluence of multiple biotic and abiotic factors as you said. But that's a great question. Any questions? Yeah. I was just wondering if you could expand a little bit on why you would think that termites are so important to the ecosystems of savannas? With your permission, I will defer the answer to that question to the next lecture, which is going to be almost entirely about that, and it's one of my favorite subjects. So, no. I mean, that's like what a great question it is that we have an entire lecture to address it. So, I think that's a perfect segue to turn the floor over to my colleague Karena.

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